FIGS. 4(a) and 4(b) are sectional views showing an embodiment of a structure of a conventional electrophoretic display apparatus. The electrophoretic display apparatus shown in these figures includes a pair of substrates 101 and 102 disposed with a predetermined spacing therebetween, an insulating liquid 103 filled in the spacing between these substrates 101 and 102, a large number of colored charged particles 104 dispersed in the insulating liquid 3, and display electrodes 105 and 106 disposed on the substrates 101 and 102, respectively, pixel by pixel. The colored charged particles 104 are electrically charged positively or negatively. The insulating liquid and the colored charged particles are colored different colors. Between adjacent pixels, a partition wall 107 is disposed so as to prevent movement of the colored charged particles 104 to another pixel, thus retaining uniform display.
By applying a voltage between the display electrodes 105 and 106, when the colored charged particles 104 are collected on the lower electrode 106, the color of the insulating liquid 103 is visually recognized as a pixel color, as shown in FIG. 4(a). When the colored charged particles 104 are collected on the upper electrode 105, the color of the charged particles 104 is visually recognized as a pixel color, as shown in FIG. 4(b). By performing such control pixel by pixel, it is possible to effect varies displays in the electrophoretic display apparatus as a whole.
In the electrophoretic display apparatus which effects display in such a principle, it is considered that it becomes possible to continuously display of image written by electric addressing without requiring display retaining electric power (memory characteristic). When the image once written by electric addressing can be retained for a long time without requiring the display retaining electric power, it is possible to minimize a power consumption necessary for image display. Further, when the displayed image is carried, it is not necessary to carry a driving apparatus and a power source, so that it becomes possible to remove only a display device portion, thus resulting in excellent portability.
As a method of providing the memory characteristic to the electrophoretic display apparatus, some methods have been proposed. For example, a thixotropic property is imparted to a migration medium by incorporating an exposure lawellar clay mineral in the migration medium. The thixotropic property in such a property that a jelly-like state is retained during rest and on the other hand, a force is exerted on a system to increase fluidity, so that the system shows a liquid-like behavior. For this reason, it is possible to hold display particles for a long time to effect stable display (Japanese Laid-Open Patent Application (JP-A) No. 2001-265261).
Further, there has been reported that electric charges are generated by acid-base dissociation between a particle surface and a resin adsorbed by the particle and a solvating effect, and a synergistic effect of dispersion stability is achieved by a steric effect of the adsorbed resin, so that it is possible to provide an image display medium which compatibly realizes long-term stability and high response speed (JP-A No. 2002-62545).
In the electrophoretic display apparatus, such a condition between a substrate surface and charged particles capable of effecting repetitive adsorption and description with respect to the substrate surface is required. More specifically, it is necessary to hold the adsorption state stably and desorb the charged particles from the substrate surface at a relatively low voltage.
Further, in order to provide a display image with the memory characteristic, it is necessary to keep stably not only the adsorption state of the charge particle to the substrate surface but also such a state that other charged particles are deposited on the charge particle absorbed to the substrate surface.
Particularly, in the case of a horizontal movement type electrophoretic display apparatus in which charged particles are moved in a substrate surface direction as described later in embodiments of the present invention, not in the case of a vertical movement type electrophoretic display apparatus as shown in FIGS. 4(a) and 4(b), a display state is formed by switching a state wherein the charged particles are extended on one of electrodes and visually recognized from a display surface side and a state wherein the charged particles are collected to the other electrode and an exposed substrate surface is visually recognized.
In this case, one of the display states is such a state that the charged particles are collected on an electrode having a smaller area when viewed from the display surface side, so that the charged particles are present on the electrode in an overlying state. Accordingly, it is necessary to keep this overlying state stably.